1. Technical Field
This invention relates generally to methods, systems and apparatus for managing digital communication systems. More specifically, this invention relates to collecting operational data from modems and/or other components in a communication system, such as a DSL system or the like, and determining FEXT interference (far-end crosstalk) between two DSL lines without requiring direct measurement of the interference. Even more specifically, at least one embodiment of this invention includes methods and apparatus for measuring NEXT interference (near-end crosstalk) between the two lines and measuring the line channel of one of the lines and thereafter using a combination of the two measurements to approximate the FEXT interference between the two lines.
2. Description of Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). In particular, asymmetric DSL (ADSL) can adjust to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions as determined during training and initialization of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line.
Currently, nationwide fixed swaths of frequency have been assigned for DSL systems, establishing static management rules governing spectra use based on worst-case, generally inapplicable transmission situations. Unreasonable static limits on and practices in DSL operation have frustrated efforts to improve service to users and to increase profitability and ubiquity for operators. Current static spectrum management attempts to ensure compatibility based on postulated, assumed and sometimes contrived binder situations. Associated with any such system is an implied probability of a given situation's occurrence. For example, crosstalk, which increases with wider spectrum use and thus higher data rates, dominates the achievable performance of DSL systems. Crosstalk models used in American National Standard T1.417-2003, “Spectrum Management for Loop Transmission Systems,” Sep. 3, 2003, American National Standards Institute are based on 1% worst-case coupling functions, also necessarily implying a probability of occurrence. Margins, which typically are targeted for 6 dB for DSL systems, are meant to protect against certain probable or improbable changes in line conditions. There also are probabilities of certain line lengths, presence of bridged taps, impulse noise, radio noise and other impairments. All these factors have been combined to generate a set of mandated spectrum masks or an equivalent set of calculated tests (called “Method B” and appearing in Annex A of T1.417-2003) for new communication technologies in an attempt to keep the probability of incompatibility below some threshold.
Measurement of crosstalk, especially FEXT interference, allows that interference to be removed from signals sent on various DSL lines. A variety of systems, techniques and methods have been developed for measuring and removing FEXT interference.
Systems, methods and techniques that provide a simple and accurate way to determine and measure FEXT between two DSL lines, without disrupting normal operation of the DSL system, would represent a significant advancement in the art. Also, systems, methods and techniques that permit measurement of FEXT interference in a DSL system from one side of the DSL loops likewise would represent a significant advancement in the art.